In recent years, development of portable communication apparatuses is proceeding remarkably, and in particular, downsizing and thickness reduction of mobile phones are in rapid progress. Other than mobile phones, downsizing and thickness reduction are in progress also for electronic apparatuses such as video cameras (handy cameras or the like), cordless telephones, and laptop personal computers (notebook-sized personal computers). Provided with a secondary battery mounted on an electronic apparatus main body, they are structured to be usable without plugging to an outlet, thereby increasing portability and convenience. At the present time, the secondary battery has a limit in capacity, and needs to be charged once every few days or every few weeks.
As charging methods, there are a contact charging method and a non-contact charging method. The contact charging method is a method in which charging is performed by making an electrode of a power receiving device and an electrode of a power feeding device to be directly contacted with each other. The contact charging method is used in general because of its simple device structure. However, electronic apparatuses are becoming lighter in weight along with the downsizing and thickness reduction of electronic apparatuses in recent years, and accordingly, a problem arises such that a contact pressure between the electrode of the power receiving device and the electrode of the power feeding device becomes insufficient, resulting in that a charging failure is caused. Further, the secondary battery is vulnerable to heat, so that a circuit thereof has to be designed to avoid overdischarging and overcharging for preventing a temperature increase of the battery. From such points, application of non-contact charging method is considered.
The non-contact charging method is a method in which a coil is provided in each of the power receiving device and the power feeding device, and charging is performed by utilizing an electromagnetic induction. Since the non-contact charging method does not have to consider the contact pressure between the mutual electrodes, it is possible to stably supply a charging voltage without being affected by contact states of the mutual electrodes. As a coil for a non-contact charging system, a structure in which a coil is wound around a ferrite core, a structure in which a coil is mounted on a resin substrate in which ferrite powder or amorphous powder is mixed, and the like are known. However, ferrite becomes fragile when processed to be thin, so that it is poor in shock resistance, and there is a problem that a trouble easily occurs in the power receiving device due to dropping of the device or the like.
Further, for reducing a thickness of a power receiving portion by corresponding to thickness reduction of an apparatus, it is considered to adopt a flat coil formed by printing a metal powder paste in a spiral shape on a substrate. However, a magnetic flux passing through the flat coil is interlinked with a substrate or the like in the apparatus, and thus there is a problem that an eddy current generated due to an electromagnetic induction causes heat generation in the apparatus. Accordingly, large power cannot be transmitted, resulting in that a charging time becomes long. Concretely, it takes about 90 minutes in a contact charging system for charging a mobile phone, but, it takes about 120 minutes in a non-contact charging system for charging the mobile phone.
The power receiving device to which the conventional non-contact charging method is applied has insufficient countermeasures for the eddy current generated by the electromagnetic induction. The power receiving device includes the secondary battery, so that it is required to suppress the generation of heat as much as possible. Since the power receiving device is attached to the electronic apparatus main body, the generation of heat exerts an adverse effect on circuit components and the like. Due to the above, it is not possible to transmit large power at the time of charging, resulting in that the charging time becomes long. Further, the generation of eddy current leads to generation of noise, and becomes a factor of reducing a charging efficiency. As a countermeasure with respect to such a point, it is proposed to provide a magnetic thin strip at a predetermined position of the power receiving device. By controlling a magnetic permeability and a sheet thickness of the magnetic thin strip, or by controlling a saturation magnetic flux density and the sheet thickness of the magnetic thin strip, the heat generation, the generation of noise, the reduction in power receiving efficiency and the like caused by the eddy current are suppressed.
There is proposed a non-contact charging method in which a magnet is arranged on a power feeding side of a non-contact charging system, to thereby perform positioning of a device on a power receiving side. For example, in WPC (Wireless Power Consortium) being an international standard, a non-contact charging system that performs positioning with the use of a magnet is disclosed in “System Description Wireless Power Transfer volume I: Low Power Part 1: interface Definition version 1.0 July 2010”.
When the positioning is performed with the use of the magnet, a magnetic saturation is caused in the conventional magnetic thin strip, resulting in that a magnetic shielding effect is drastically reduced. For this reason, there is a concern that a temperature increase of a secondary battery is caused at the time of charging, and a cycle life of the secondary battery is reduced. A conventional magnetic shield has a magnetic thin strip whose saturation magnetic flux density is 0.55 to 2 T (5.5 to 20 kG), for example, and is formed of one piece of such a magnetic thin strip or stacked magnetic thin strips as above whose number is in a range of 3 or less. Even if a stack of the magnetic thin strips is used as the magnetic shield, there is a possibility that the magnetic shield easily causes a magnetic saturation due to a magnetic field generated from the magnet arranged on the power feeding device, resulting in that a function as the magnetic shield cannot be exhibited.